The present invention relates to a compatible television receiver which can visually reproduce a television signal of a standard television system and a television signal of a second television system having a horizontal scanning frequency n times that of the television signal of the standard television system.
The standard television systems now employed are typically an NTSC system, a PAL system, and a SECAM system. In the NTSC system, for example, the number of the horizontal scanning lines is 525 and the number of frames per second is 30. In an enlarged picture size of the recent television set of any system, the scanning line intervals are widened and distinctive on the screen, thus providing a coarse picture. Particularly in present times a large sized picture is often required and the development of a high resolution picture television signal is urgently required for improving the poor resolution in the television picture. For this reason, a high definition television system (HDTV) having a number of the scanning lines two to three times that of the currently used standard television system and with a frequency band-width 5 to 10 times that of the standard television system, will be employed in the near future. The HDTV already proposed by NHK (Nippon hoso kyokay, the Japanese national broadcasting corporation) has 1125 scanning lines and a 20 MHz luminance band-width. The HDTV processed by the EIA(Electronic Industries Association) has 1023 scanning lines and a 21.1 MHz luminance band-width. The HDTV proposed by the BBC has 1501 scanning lines and a 50 MHz luminance band-width.
Those HDTVs are now being discussed by a working group in SMPTE (Society of Motion Picture and Technical Engineers), which has been specially set up for the realization and execution of HDTV. It is believed that the realization of a HDTV television set, which can also be adaptable for the standard television system, that is, a compatible television set, will promote the prevalence of the HDTV, particularly in a transition present transient period to the HDTV days.
In recent times, there has been an increasing demand for character display systems for displaying alphanumeric characters, symbols and the like, which are used in the terminal equipment of the computer, and a graphic display for displaying graphs, patterns and the like. In some display devices of this type, in order to improve the amount of information handled, the luminance signal band-width is widened and the number of scanning lines is increased to about two times that of the standard television system. If those display devices are so designed as to visually reproduce a television signal of the standard television system, the display device can display the signals from VTR or cameras, thus finding a wide use. In this respect, it is desired to realize such compatible display devices as soon as possible.
In realizing compatible television receivers, a problem is encountered in that it is difficult to use a deflection circuit common for two the television systems for their signal processing, since the horizontal scanning frequencies of the two television systems are greatly different from each other.
Two methods to cope with this problem have been known. One of those methods is the horizontal deflection circuit is partially switched by means of a relay or the like so that it operates at the scanning frequency of either the standard television system or the second television system. The method, however, has a drawback of insufficiency in reliability, since relay means must switch-over circuit constants of a high potential circuit portion such as a resonance capacitor, an S distortion correcting capacitor, and a linearity control coil.
The second method is that, as in the standard television system converting method between the NTSC, PAL and SECAM system, a video signal of one television system is converted into a signal with a horizontal scanning frequency of the other television system, and the operating frequency of the deflection circuit is fixed to the scanning frequency of the other television system.
However, the second method requires a frame memory and yet a large memory capacity. Furthermore, this method is applicable only for the conversion between two television systems of which the scanning frequencies are in a fixed relation. In this respect, this method has a poor flexibility in converting the television signals of the television systems one to another.